Food for the deep sea

The input of particulate organic matter to the seafloor is the main food and energy source for heterotrophic benthic organisms; the quantity and quality of this material (comparably "fresh" marine or refractory terrigenous material) is crucial for deep-sea organisms. Two major processes are responsible for food supply to deep-sea benthos: export from the photic zone and lateral advection in nepheloid layers. Both processes do not supply organic matter at constant rates rather a pulse-like supply is to be anticipated.

At HAUSGARTEN observatory, flux assessments are conducted by means of sediment traps. Measurements reveal a bimodal seasonal pattern with elevated export from the upper layer during May/June and August/September. Annual flux of biogenic matter in the area is almost constant with 8.5 - 8.8 g m-2, whereas the flux of total matter varies from 13 - 32 g m-2, seemingly influenced by the ice condition in the region. To track possible sources of laterally advected organic and inorganic matter, we perform mineralogical analyses of the lithogenic fraction of samples from sediment traps and bottom sediments.

The export of organic material to the deep seafloor is governed by the activity of consumers in the upper twilight zone (100 - 500 m), where the majority of organic carbon is recycled. The material reaching the seafloor is oxidized by the benthic biota trough a vertical redox cascade. The reduced products from anaerobic degradation are ultimately re-oxidized by an equivalent amount of O2, and consequently, the benthic O2 uptake is a commonly used measure for the total benthic mineralization rate. Benthic O2 consumption is thus used (1) for aerobic heterotrophic activity of bacteria and fauna, and (2) for the re-oxidation of reduced inorganic products released during the anaerobic heterotrophic degradation. Biogeochemical processes in marine sediments consequently play a key role in understanding the budget of the marine and global carbon cycle.